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This is a question we've wondered about for some time.  After all, they can somehow distinguish between parthenocarpic fruit, which are low in furanocoumarins and the preferred food of webworms, and normal fruit.
Yet when we've augemented plant parts with furanocoumarins (by painting them on in solvent) and allowed webworms to choose between these parts and otherwise identical parts, they seem incapable of distinquishing a difference. 

The key, we think, lies in other differences between parthenocarpic and normal fruit.    What distinguishes these two?  Of 31 components detected by gas chromatography, 21 differed significantly.   Moreover, all 21 compounds are significantly positively correlated with furanocoumarin content.  Among the most highly correlated, however, was octyl-butyrate.  Ninety percent of the variation in total furanocoumarin content can be predicted by octyl-butyrate content

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Octyl-butyrate is both an olfactory and feeding deterrent to webworms. That's a convenient relationship because it might allow webworms to indirectly assess the amounts of furanocoumarins among its food choices without first ingesting large amounts.